A compact disc (hereinafter, abbreviated to CD) has been spread as an optical disc. In the CD, audio data is sequentially divided into blocks, subjected to an encoding of an error correction code, and after that, and EFM (Eight To Fourteen) modulated, and a modulation result is recorded by an NRZI (Non Return to Zero Inverted) modulation.
As a result of the EFM modulation, for a basic period T serving as a period of a channel clock, the audio data is recorded onto the disc by a repetition of pits and a land having nine kinds of lengths 3T to 11T in which the basic period T is used as a unit. In case of a CD, each pit has a length of about 0.87 to 3.18 [μm] in correspondence to 3T to 11T, a pit width of about 0.5 [μm], and a depth of about 0.1 [μm].
The audio data recorded on the CD is 2-channel data in which a sampling frequency is equal to 44.1 [kHz] and the number of quantization bits is equal to 16 bits. However, it is also demanded to realize a high sound quality and multichannels. In this case, it is necessary that there is reproducing compatibility in which the audio data can be reproduced by the existing CD player. It is not preferable that a time of an audio program which can be recorded on one CD becomes short due to the realization of the high sound quality and multichannels. Further, since a copy preventing technique for copyright protection is not used in the CD, in a present situation, data is illegally copied.